CT colonography: avoiding traps and pitfalls.

Abstract

Computed tomographic colonography (CTC) is a reliable technique for detecting tumoral lesions in the colon. However, good performance of polyp detection is only achieved if experienced CTC radiologists combine meticulous interpretation with state-of-the-art CTC technique. To reach this experience level, CTC training is mandatory. With a considerably long and steep learning curve, it has been demonstrated that in inexperienced hands both technical failure and observer errors stand for the majority of missed lesions. The purpose of this pictorial review is to give an overview of traps and pitfalls in CTC imaging resulting in false negative and positive findings, and how to avoid them by application of state-of-the-art CTC technique and interpretation.

a Three-dimensional view of the rectum showing a small polypoid defect surrounded by some fluid (white arrowhead). b The corresponding axial image in the supine position shows a negative filling defect floating in a contrast material pool (white arrowhead). White arrow hyperdense tip of the rectal catheter. c In the prone position the filling defect has moved to the anterior part of the rectum (white arrowhead). Both positional shift and floating of the defect in contrast material allow for a correct diagnosis of non-tagged stool

a Three-dimensional view of the ascending colon showing a longitudinal luminal defect along a semi-lunar fold (black arrow): flat lesion? b, c The corresponding axial and sagittal views show a subtle stair step artefact on the abdominal wall (white arrowheads) and a doubled or “twin fold” aspect of the semilunar fold explaining the small longitudinal defect as a subtle breathing artefact

a Axial view of the caecum in the supine position showing an 8-mm polyp on the left lateral side (white arrowhead). b The corresponding view in the prone position shows the same defect on the anterior caecal wall, submerged in slightly tagged fluid (white arrowhead). Residual stool with positional shift? c, d There is a rotation of the caecum over 90° as is shown by the different positions of the ileo-caecal valve (white arrowhead) and the terminal ileum (white arrow) in both acquisitions: image of true polyp or pseudo-stool due to segmental mobility

a Three-dimensional view of the sigmoid in a patient with dolichocolon: image suggestive of large polypoid mass (black arrowhead). b Corresponding sagittal view in an abdominal window setting showing an acute flexure of the sigmoid over 180° with thickening of the colonic wall and compression of the inner structures. c The corresponding 3D view in the prone position shows a normal fold

a Three-dimensional view at the hepatic flexure showing a tumoural mass with overhanging edges (black arrow). There is a subtle bump on the colonic wall more proximally (white arrow). b The corresponding 2D image shows the large tumour with overhanging edges and shoulder formation (black arrow). The subtle bump was caused by a small pericolonic nodule (white arrow) suggestive of mesenteric metastasis

a The rectum and its “peri-catheter” segment need special attention! Three-dimensional view of the anal margin shows interpretation is hampered by the rectal catheter (black arrow). The impression of the inflated balloon on the rectal wall is nicely appreciated (white arrows). A flat luminal defect is detected at the level of the anal margin, abutting the rectal catheter (white arrowhead). b, c The corresponding prone view after deflation of the balloon of the rectal catheter shows a large sessile polypoid defect (white arrowhead) against the rectal catheter (black arrow) confirmed on 2D as a solid lesion >10 mm, prompting optical colonoscopy

a Three-dimensional view of the sigmoid showing a polypoid defect (white arrow). b The corresponding axial image shows that the defect is caused by a nodular structure consisting of a hypodense centre and peripheral hyperdense ring typical of a diverticular faecalith (white arrow)

a Three-dimensional view of the ascending colon in the supine position showing a 9-mm polypoid defect (white arrow). b The corresponding 2D view confirms the probable polyp (white arrow). c However, the prone acquisition reveals a diverticulum at the same location (white arrow). Review of the 2D image in the abdominal window shows a central hypodensity of -100 to 0 HU, consistent with a subtle amount of fat. These two findings confirm the diagnosis of inverted diverticulum